Raman spectroscopy is a certain spectroscopy technique that enables observation of vibrational, rotational, and other low frequency modes in a system. Raman spectroscopy is common practice in e.g. in the field of chemistry in order to provide a fingerprint by which different molecules may be identified.
Raman spectroscopy typically provides information about molecular vibrations that may be used for identification or quantification of a material which a sample is made up of. The sample is typically illuminated by an electromagnetic radiation source such as a laser or LED having monochromatic light. The light incident on the sample is scattered, absorbed or transmitted. The majority of the scattered light is typically in the same frequency as the light source, this is typically known as Rayleigh or elastic scattering i.e. the energy is conserved. Typically, a very small amount, typically in the range of 10−5% of the incident light intensity, is scattered by Raman scattering or inelastic scattering, i.e. the energy is not conserved resulting in a shift in wavelength of the scattered light.
A Raman spectrum of the sample may be obtained by plotting the intensity of the shift of wavelength versus the frequency.
Raman spectroscopy is a well-established spectroscopic technique for rapid identifications of chemicals with high degree of accuracy. Every material will give rise to a unique Raman spectrum, which is why the technique is suitable for identifying materials.
The Raman effect typically occurs when focused monochromatic (laser) light interact with vibrational modes of molecules comprised in a sample are illuminated. Light scattered from the molecules gives rise to a vibrational spectrum that typically consists of a series of lines constituting a molecular “fingerprint”.
Raman instruments typically are common in pharmaceutical laboratories.
The continued miniaturization of electronic components has furthermore led to the production of portable and handheld Raman instruments used for hand measurements of liquids, powders and solids.
The optical module of Raman instruments generally consists of three main parts: an excitation source, e.g. typically a laser, a sampling device, e.g. typically an optical probe, and a spectrometer or detector. Optical probes typically deliver the laser radiation to the sample and transmit back-scattered Raman radiation from the sample to the spectrometer.
Typically, there are three general types of the optical probe geometries, remote sampling probes using fiber optics, conventional sampling probes with free-space radiation path and Raman imaging probes—which is a combination of a Raman spectrometer with a microscope.
The first reported Raman probe was a remote sampling probe by McCreery et. al in U.S. Pat. No. 4,573,761. The probe head was a bundle of three optical fibers where the central fiber was used to deliver laser beam to the sample and two others to collect Raman light from the sample. The laser beam was divergent and the efficiency of Raman photons was poor. Many improvements to the McCreery probe were done to increase collection efficiency.
Conventional sampling probes may typically remove the fiber-background and optimize light throughput by integrating a lens system and a light-filtering material on the free-space laser and collection paths. These probes typically use 180° or 90° geometries.
There are several variations to both geometries. In some applications the angle between the laser and collection axis exceeds or is less than 90°, whereas some applications use parallel axis for excitation and collection radiation.
The 180° probe configuration has become quite common in commercial Raman instruments and has many advantages. For example, the working distance between the collection lens and the sample can be up to several centimeters, which typically makes Raman sampling through a vial possible. Since laser and collection light pass through the same surfaces, curved vials or bottles can be tolerated.
However, the sample alignment is a common problem for both geometries because the laser focus position on the sample is strongly affected by the Raman signal strength.
Different sampling geometries have different sensitivity to the focus. For example, the Raman microscopes obtain spectra from a very small sample region (a few microns in depth and 1 μm laterally) and is typically extremely sensitive to focus position—a few microns of motion along the optical axis can reduce signal by half or more.
US 2014/0221753A1 discloses an objective lens arrangement for confocal endomicroscopy. Here, an imaging arrangement can be configured to generate a microscopic image of the anatomical structure(s), wherein the imaging arrangement can include a variable focus lens, and can be provided in the housing arrangement.
Furthermore, the Raman spectrometer sensitivity is related to the spot size and to the sampling area size. A small focus spot is desired to achieve high sensitivity but this also reduces the sampling area, which is an issue for non-uniform or non homogenous samples. This problem has been addressed by rooter- and beam scanning techniques such as described in U.S. Pat. No. 8,310,669 and/or US20120162642A1.
A typical problem for all conventional probes is stray light caused by the inner mechanical structure of the probes.
The Raman spectroscopy is based on inelastic scattering, i.e. the kinetic energy of an incident particle is not conserved, or on Raman scattering, i.e. the inelastic scattering of a photon. The scattering is typically induced by light in the form of a laser beam typically in the visible, near infrared, or near ultraviolet range.
Typically, a sample is illuminated with a laser beam having high laser power density. The electromagnetic radiation form the illuminated spot of the sample is the collected with a lens and sent through a monochromator. Typically, elastic scattered radiation is filtered out, while the rest of the collected light is dispersed onto a detector through a filter.
The high laser power density typically used in Raman spectroscopy provides problems with the technique. As it is common to illuminate small areas of a sample in Raman spectroscopy the high laser power density typically leads to massive heat development in the sample which may severely damage the sample. Putting a large amount of energy into the sample could also lead to other dangers, e.g. in the case of potentially explosive substances.
Another typical problem that limits the use of Raman spectroscopy in investigation of colored samples is fluorescence and Raman emission from the fiber core itself that hide the, in this context, very weak Raman signal.
Another problem is that especially black and brown-colored samples experience localized heating if the laser power density is too high. This can generally be observed in Raman spectra as a broad sloping background overlying the Raman spectrum because of blackbody radiation making it difficult to actually discern the scattering from surrounding noise.
Thus, there is a need for new and improved systems for achieving Raman spectroscopy.